1. Field of the Invention
The present invention relates to chip-type piezoelectric filters for use as band-pass filters, and also relates to manufacturing methods therefor. More particularly, the present invention relates to a piezoelectric filter having a construction such that a plurality of piezoelectric substrates having energy-trap filter units disposed thereon, are laminated with adhesive layers disposed therebetween, and to a method of manufacturing such a piezoelectric filer.
2. Description of the Related Art
Conventionally, various kinds of chip-type piezoelectric filters having a construction such that a plurality of piezoelectric substrates, on which piezoelectric filter units are constructed, are laminated with adhesive layers therebetween, have been suggested in, for example, Japanese Unexamined Patent Application Publications Nos. 10-335976, 10-135769, 11-68509, among others.
FIG. 5 is an exploded perspective view of an energy-trap piezoelectric filter 101 which is disclosed in Japanese Unexamined Patent Application Publication No. 11-68509.
According to this publication, the energy-trap piezoelectric filter 101 includes a first piezoelectric substrate 102 and a second piezoelectric substrate 103. The first piezoelectric substrate 102 is provided with a first piezoelectric filter unit 104 and a transfer capacitor unit 105, and the second piezoelectric substrate 103 is provided with a second piezoelectric filter unit 106 and a transfer capacitor unit (not shown). The shapes of electrodes disposed on the bottom surface of the first piezoelectric substrate 102 are approximately the same as the shapes of electrodes disposed on the top surface of the second piezoelectric substrate 103. In addition, the shapes of electrodes disposed on the bottom surface of the second piezoelectric substrate 103 are approximately the same as the shapes of the electrodes disposed on the top surface of the first piezoelectric substrate 102.
The first and the second piezoelectric substrates 102 and 103 are laminated with an insulating spacer 107 therebetween, and insulating spacers 108 and 109 are laminated on the outwardly facing surfaces of the piezoelectric substrates 102 and 103, respectively. In addition, case substrates 110 and 111 are laminated on the outwardly facing surfaces of the spacers 108 and 109, respectively. The chip-type piezoelectric filter 101 is formed by laminating the above-described components, and by forming external electrodes on the outwardly facing surfaces of the resulting laminate.
The spacers 107 to 109 have through holes 107a to 109a, respectively, which provide sufficient spaces for the piezoelectric filter units 104 and 106 constructed on the piezoelectric substrates 102 and 103, respectively, to freely vibrate therein. The spacer 107 is provided with notches 107b to 107d at the periphery thereof, and the spacers 108 and 109 are provided with notches 108b and 109b, respectively. These notches 107b to 107d, 108b, and 109b are filled with a conductive material of which the external electrodes are formed. More specifically, the external electrodes are formed by applying the conductive material on the outwardly facing surfaces of the laminate including the above-described components, in a manner such that the conductive material also fills the notches 107b to 107d, 108b, and 109b. Accordingly, the reliability of the electrical connection is improved.
Similar to other electronic components, there is an increasing demand to reduce the size, especially the height, of the conventional chip-type piezoelectric filter shown in FIG. 5.
To realize a reduction in the height, the thicknesses of the spacer 107 disposed between the piezoelectric substrates 102 and 103 and of the spacers 108 and 109 laminated on the outwardly facing surfaces thereof must be reduced. In the conventional chip-type piezoelectric filter 101, however, there is a limit to the amount of reduction in the thicknesses of the spacers 107 to 109 or of an insulating adhesive, which may be used in place of the spacers 107 to 109. Accordingly, it has been difficult to realize a reduction in the height of such components.
In order to overcome the problems described above, preferred embodiments of the present invention provide an energy-trap piezoelectric filter which is easily made much thinner than conventional devices, and in which the reliability of electrical connection between electrodes is greatly improved and highly reliable. In addition, preferred embodiments of the present invention provide a method of manufacturing the energy-trap piezoelectric filter.
According to a preferred embodiment of the present invention, an energy-trap piezoelectric filter includes a first piezoelectric substrate having a first piezoelectric filter unit and a first transfer capacitor unit, a second piezoelectric substrate having a second piezoelectric filter unit and a second transfer capacitor unit, an adhesive layer which is disposed between the first and the second piezoelectric substrates so that a laminate including the first and the second piezoelectric substrates and the adhesive layer is defined, and a plurality of external electrodes disposed on the outwardly facing surfaces of the laminate. The elasticity modulus of the adhesive layer is preferably in the range of about 0.5 MPa to about 3000 MPa, and the adhesive layer is provided with at least one notch at the edge thereof. In addition, a material of which the external electrodes are formed is inserted into the at least one notch.
The adhesive layer may have a through hole that is arranged to provide a sufficient space for the first and the second piezoelectric filter units to freely vibrate therein.
In addition, the adhesive layer may be constructed of a thermosetting adhesive sheet that is formed in advance.
Preferably, the notch is filled with the material from which the external electrodes are formed.
According to another preferred embodiment of the present invention, a method of manufacturing a piezoelectric filter includes the steps of preparing a first piezoelectric substrate, which includes a first piezoelectric filter unit and a first transfer capacitor unit, and a second piezoelectric substrate, which includes a second piezoelectric filter unit and a second transfer capacitor unit, forming a laminate by laminating the first and the second piezoelectric substrates with an adhesive therebetween, and forming a plurality of external electrodes on the outwardly facing surfaces of the laminate. The elasticity modulus of the adhesive after being cured is preferably in the range of about 0.5 MPa to about 3000 MPa. In addition, the adhesive which is disposed between the first and the second piezoelectric substrates is provided with at least one notch at the edge, and the external electrodes are formed in a manner such that a material from which the external electrodes are formed is inserted into the notch.
A thermosetting adhesive sheet which is formed in advance and which is cured by heat may be used as the adhesive.
Preferably, the thermosetting adhesive sheet is not adhesive at room temperature and becomes adhesive when heat is applied in an adhering process, and does not have fluidity.
In addition, the adhesive may have a through hole that is arranged to provide a sufficient space for the first and the second piezoelectric filter units to freely vibrate therein.
According to the piezoelectric filter of various preferred embodiments of the present invention, the elasticity modulus of the adhesive layer is in the range of about 0.5 MPa to about 3000 MPa. Thus, even when the thickness of the adhesive layer is reduced, undesired spurious responses are effectively suppressed and minimized. Moreover, at least one notch is formed at the edge of the adhesive layer, and the material from which the external electrodes are formed is inserted therein. Accordingly, even when the adhesive layer is thin, the connection area between the side surfaces of the adhesive layer and the external electrodes is greatly increased, and the reliability of the external electrodes is significantly enhanced.
When the adhesive layer has a through hole which provides a sufficient space for the first and the second piezoelectric filter units to freely vibrate, stable filter characteristic are obtained.
In addition, when the adhesive layer is constructed of a thermosetting adhesive sheet which is formed in advance, the first and the second piezoelectric substrates are easily laminated by disposing the adhesive sheet therebetween and by applying heat to cure the adhesive sheet.
In addition, when the notch is filled with the material from which the external electrodes are formed, reliability of the external electrodes is further improved.
According to a method of manufacturing a piezoelectric film of various preferred embodiments of the present invention, the elasticity modulus of the adhesive that is disposed between the first and the second piezoelectric substrates is in the range of about 0.5 MPa to about 3,000 MPa. Thus, even when the thickness of the adhesive layer is reduced, undesired spurious responses are effectively be suppressed. In addition, since at least one notch is formed on the edge of the adhesive, the reliability of the external electrodes is greatly improved by inserting the material from which the external electrodes are formed into the notch.
When the thermosetting adhesive sheet that is formed in advance and is cured by heat is used as the adhesive, productivity is greatly improved because of its handiness in the laminating process.
In addition, when the thermosetting adhesive sheet is not in adhesive state under room temperature and becomes adhesive when heat is applied in the adhering process, and does not have fluidity, the adhesive sheet is easily disposed between the first and the second piezoelectric substrates. Accordingly, the adhesive layer is more reliably formed at a desired position by applying heat.
In addition, when the adhesive sheet has a through hole which provides a sufficient space for the first and the second piezoelectric filter units to freely vibrate therein, stable filter characteristics are obtained by the vibrations of the first and the second filters.
Other features, elements, characteristics and advantages of the present invention will become apparent from the following detailed description of preferred embodiments of the present invention with reference to the attached drawings.